Optical disks include two types in terms of recording manner: one is a data reproducing-only optical disk, and the other is a data recording/reproducing optical disk, such as a rewritable optical disk or a write-once optical disk. A typical example of the reproducing-only optical disk is a compact disk, from which pre-recorded data in the form of concave and convex pattern of pits are reproduced as a light beam detects the pits. On the other hand, the data are reproduced from the recording/reproducing optical disk through the magneto-optical effect or physical characteristics change (for example, change in reflectance) caused by the crystal-amorphous phase change.
Newly developed is a hybrid optical disk having both the data reproduce-only area and data recordable area in a single disk. For example, as shown in FIG. 22, an inner track area of an optical disk 100 is allocated as a data reproduce-only area 101, and an outer track area is allocated as a data recordable area 102. As shown in FIGS. 23(a) and 23(b), pits 103 (depth: d1, width: w11) are pre-formed in the data area 101. On the other hand, as shown in FIGS. 24(a) and 24(b), grooves 105 (depth: d2, width: w21) and lands 106 (width: w22) are formed on the data area 102.
A modulation degree of the above hybrid optical disk varies considerably depending on the kinds of signals. To be more specific, compared with a reproduction signal from the data recordable area where a magneto-optical signal or phase change signal is recorded, a reproduction signal from the data reproduce-only area made of a concave and convex pattern of pits has a higher modulation degree and larger amplitude. For this reason, a high-quality signal is reproduced even from shorter pits.
FIG. 25 shows a relation of a tracking error signal (push-pull signal) amplitude versus an optical depth of the groove on the optical disk, and a relation of a reproduction signal (pit signal) amplitude versus an optical depth of the pit on the optical disk. Let a wavelength of a light beam be .lambda., then an amplitude of the push-pull signal reaches its maximum when the groove depth is .lambda./8, and the push-pull signal reaches at least half the maximum amplitude when the groove depth is .lambda./20-.lambda.5. An amplitude of the pit signal reaches its maximum when the pit depth is .lambda./4, and the pit signal reaches at least half the maximum amplitude when the pit depth is .lambda./8-.lambda./4. Although it is not illustrated in the graph, the pit signal also reaches at least half the maximum amplitude when the pit depth is .lambda./4-.lambda..times.3/8%, and therefore, the pit signal reaches at least half the maximum amplitude when the pit depth is .lambda./8-.lambda..times.3/8. The larger the signal amplitude, the more stable the tracking servo or signal reproduction. Therefore, the target optical depths of the groove and pit during the processing step are .lambda./8 and .lambda./4, respectively. The optical depth divided by the substrate's refraction factor n, d1=.lambda./(4.multidot.n), d2=.lambda./(8.multidot.n) are the depths actually made in the substrate.
The data are recorded through a so-called CLV (Constant Linear Velocity) system. To be more specific, a rotation synchronizing signal is imbedded in the disk, so that when the data are reproduced, the rotation of the disk is controlled in such a manner that the rotation synchronizing signal is in sync with a rotation reference signal indicating a disk rotation speed reference. The rotation synchronizing signal can be imbedded into the disk by more than one methods. For example, the rotation synchronizing signal is imbedded into the concave and convex pattern of pits formed on the track, or wobbled guiding grooves called wobbles formed on the track. Japanese Laid-open Patent Application No. 338066/1994 (Tokukaihei No. 6-338066) discloses an optical disk having guiding grooves composed of grooves whose wobbling frequency switches per rotation and a pit array indicating the switching of the wobbling frequency. In this optical disk, spaces between adjacent guiding grooves are composed of a pit array used as the data reproduce-only area. The above reference discloses another type of optical disk, in which both the side walls of the guiding grooves are wobbled at different frequencies. The rotation synchronizing signal is imbedded at the time the side walls are wobbled.
Further, the above reference discloses a manufacturing method of an optical disk, by which pits having semicircular edge portions with a diameter as long as the pit width are formed using a recording light beam having a spot of substantially the same size as the guiding groove.
To meet the increasing demand for a higher density optical disk, Japanese Laid-open Patent Application No. 314538/1993 (Tokukaihei No. 5-314538) discloses a method of increasing density of a data recording/reproducing optical disk, in which data can be recorded in both the grooves and lands. To be more specific, as shown in FIGS. 26(a) and 26(b), an optical disk 120 includes a substrate 122 having thereon concave grooves 123 and the remaining convex lands 124 on its back surface 122b. A light beam is incident on the main surface 122a of the substrate 122, and converged onto either the groove 123 or land 124 to record, reproduce, or erase the data. Thus, in this case, a pair of the groove 123 and land 124 forms a guide track 125, and an interval between two adjacent guide tracks 125 is defined as a track pitch.
The side wall 128a of the groove 123 wobbles in accordance with address information, while the other side wall 128b does not. The grooves 123 and lands 124 are designed to have an equal average width. A recording film 126 is sputtered to the back surface 122b of the substrate 122 along its concave and convex pattern. In case of a reflection optical disk, a reflection film 127 is further formed atop of the recording film 126. The wobbles thus made make it possible to detect addresses from both the groove 123 and land 124. However, the reference is silent about the hybrid optical disk.
Japanese Laid-open Patent Application No. 7339/1996 (Tokukaihei No. 8-7339) discloses a hybrid optical disk, in which the guiding grooves (pit-on grooves) composed of grooves having pits thereon are used as the data reproduce-only area, and spaces between the guiding grooves are used as the data recordable area, so that the above two kinds of areas alternate on the disk.
Additionally, shortening pits or recording marks along the track or narrowing a track pitch can also increase density of an optical disk.
The hybrid optical disk 100 of FIGS. 22 through 24(b) has both the data reproduce-only area and data recordable area. However, as shown in FIGS. 23(a) and 23(b), spaces 104 (width: w12) between the adjacent pits 103 in the data reproduce-only area 101 are left as a dead area, thereby making it difficult to increase a recording capacity of the data reproduce-only area 101.
In addition, since the conventional optical disk 100 (see FIGS. 22 through FIG. 24(b)) has the pits 103 (depth: d1, width: w11) and grooves 105 (depth: d2, width: w21) in different concave shapes, the manufacturing process of the optical disk is complicated.
Further, to attain at least half the maximum signal amplitude, the pits and grooves in the optical disk 100 are made to have the target optical depths in a range between .lambda./8 and .lambda..times.3/8 and in a range between .lambda./20 and .lambda./5, respectively. However, since the pits and grooves match neither in the depth range nor width, it is very difficult to manage the shapes of the pits and grooves.
As has been explained, the above referred Japanese Laid-open Patent Application No. 338066/1994 (Tokukaihei No. 6-338066) discloses a hybrid optical disk having guiding grooves composed of grooves whose wobbling frequency switches per rotation and a pit array indicating the switching of the wobbling frequency, in which spaces between the adjacent guiding grooves are composed of a pit array used as the data reproduce-only area. This type of optical disk also demands the pit array and grooves in different concave shapes. Thus, like the above case, the manufacturing process is complicated.
In addition, as has been explained, the above-referred Japanese Laid-open Patent Application No. 7339/1996 (Tokukaihei No. 8-7339) discloses a hybrid optical disk, in which the guiding grooves (pit-on grooves) composed of grooves having thereon the pits are used as the data reproduce-only area, and spaces between the adjacent guiding grooves are used as the data recordable area, so that the above two kinds of areas alternate on the disk. This type of optical disk also demands the pits and grooves in different concave shapes and the manufacturing process is complicated as well as the above two cases.
Further, address information is preformatted into this type of optical disk in the concave and convex pattern of pits made on the guiding grooves, and no address information can be detected from the spaces between the adjacent guiding grooves. Thus, to record/reproduce the data into/from the spaces between the adjacent guiding grooves (the data recordable area), the address information must be detected from the guiding grooves (data reproduce-only area). Therefore, 2-beam optical head is indispensable to irradiate light spots respectively on the guiding groove and space between the adjacent guiding grooves concurrently, and an address information detecting signal processing system must be switched from the guiding grooves to the spaces between the adjacent guiding grooves and vice versa. This complicates the arrangement of the optical disk recording/reproducing apparatus.